1. Field of the Invention
The invention relates generally to seismic prospecting. More particularly, the invention relates to seismic sources for generating acoustic signals in water for marine seismic surveys.
2. Background of the Technology
Scientists and engineers often employ seismic surveys for exploration, archeological studies, and engineering projects. In general, a seismic survey is an attempt to map the subsurface of the earth to identify formation boundaries, rock types, and the presence or absence of fluid reservoirs. Such information greatly aids searches for water, geothermal reservoirs, and mineral deposits such as hydrocarbons. Petroleum companies frequently use seismic surveys to prospect for subsea hydrocarbon reserves.
During a subsea or marine seismic survey, an acoustic energy source, also referred to as a “seismic energy source” or simply “seismic source,” is introduced into the water above the geologic structure of interest. In general, seismic sources can provide single, discrete pulses of seismic energy or continuous sweeps of seismic energy. Both types of seismic sources generate seismic energy signals or waves (i.e., pulse of acoustic energy) that propagate through a medium such as water or layers of rocks. In marine applications, each time the seismic source is triggered, it generates a seismic energy signal that propagates down through the water and the water-sea floor boundary into the subsea geological formations. Faults and boundaries between different formations create differences in acoustic impedance that cause partial reflections of the seismic waves. These reflections cause acoustic energy waves to return toward the water, where they may be detected at the seafloor by an array or set of ocean-bottom geophones or other seismic energy receivers, or detected within the water layer by an array or set of spaced hydrophones or other seismic energy receivers. The receivers generate electrical signals representative of the acoustic or elastic energy arriving at their locations.
The acoustic or elastic energy detected by the seismic receivers is generally amplified and then recorded or stored in either analog or digital form. The recording is made as a function of time after the triggering of the seismic energy source. The recorded data may be transported to a computer and displayed in the form of traces (i.e., plots of the amplitude of the reflected seismic energy as a function of time for each of the geophones or seismic energy receivers). Such displays or data subsequently undergo additional processing to simplify the interpretation of the arriving seismic energy at each receiver in terms of the subsurface layering of the earth's structure. Sophisticated processing techniques are typically applied to the recorded signals to extract an image of the subsurface structure.
There are many different methods for producing acoustic energy waves or pulses for seismic surveys. Conventional seismic surveys typically employ artificial seismic energy sources such as explosives (e.g., solid explosives or explosive gas mixtures), shot charges, air guns, or vibratory sources to generate seismic waves. Some of these approaches provide for strong acoustic waves, but may be harmful to marine life and/or incapable of limiting the generated acoustic waves to desired frequencies. A more controllable technique for producing acoustic waves is to employ a subsea or marine reciprocating piston seismic source. Such devices typically rely on a piston that acts against the water to generate extended-time acoustic energy frequency sweeps. The piston is usually driven by a linear actuator, a voice coil, or a piezoelectric crystal transducer. The piston may be directly driven, with the motion of the piston almost entirely constrained, or may resonate by balancing water forces against a tunable spring, with the driving force only “topping up” the energy lost to the water. The piston may also be partially constrained and partially allowed to undergo a controlled resonance. The tunable spring may be, for example, a mechanical spring, a regenerative electromagnetic inductive device, an air spring, or a combination of these.
FIG. 1 illustrates an example of a conventional reciprocating piston marine seismic source 10 disposed in water 12 beneath the sea surface 11. Source 10 includes a cylinder 15 having a central axis 19 and a piston 20 coaxially disposed in the cylinder 15. The lower end 15a of cylinder 15 is open to the water 12, and the upper end 15b of the cylinder 15 is sealed or closed off from the water 12 with a cap 16. Piston 20 sealingly engages cylinder 15, thereby defining a volume 17 within cylinder 15 that is filled with a compressible gas such as nitrogen or air. The piston 20 has a flat or planar face 20a that faces and operates against water 12 in the lower end 15a of cylinder 15 and a flat or planar face 20b that faces the air in volume 17. Piston 20 is coupled to a linear actuator 25 disposed in volume 17 with a shaft 21. The linear actuator 25 is held in position relative to the cylinder 15 by support members 26. Piston 20 axially oscillates within cylinder 15 under the control of the linear actuator 25. As piston 20 reciprocates within cylinder 15, face 20a acts against water 12 in lower end 15a to generate acoustic energy waves that propagate downward through the water 12.
Without being limited by this or any particular theory, axially reciprocating piston 20 solely with actuator 25 requires impractically large amounts of power. Therefore, in many cases, a tuned system (e.g., tunable spring) that resonates the piston at the desired output frequency is often employed, thereby reducing the total input power requirements. However, this solution has two disadvantages. First, the energy must be input during the active sweep (i.e., the phase in which the acoustic energy waves of a desired frequency or desired frequency range are being generated by the seismic source), which may be of a relatively brief duration compared to the time period between active sweeps. In general, the shorter the time period over which a given amount of energy is input, the greater the power requirements. Second, the energy must be added in a carefully controlled fashion such that it does not disrupt the resonance, and this must be done even as the resonant frequency changes as the device performs a sweep.
At higher frequencies and shallow water depths, an oscillating-piston seismic source may produce cavitation—a phenomenon that occurs when the local static pressure head minus the local vapor pressure head becomes less than the local piston-velocity head for some point on the piston face. When cavitation occurs, the seawater temporarily decouples from the moving piston face, leaving a vacuum adjacent to that part of the piston face. The vacuum then collapses violently, possibly damaging the piston face in the process. In addition, the abrupt collapse produces undesirable turbulence, which dissipates energy uselessly as heat instead of as acoustic radiation.
Accordingly, there remains a need in the art for marine seismic sources that produce energy in a controlled frequency sweep that is extended in time, without any impulsive shocks, and to produce energy only in the frequency bands of interest so that only the minimum necessary peak power is emitted at each frequency and all the energy emitted is useful. Such sources would be particularly well received if they can produce energy at frequencies lower than about 8 Hz, which has proven to be difficult to achieve to date using conventional seismic sources.